As for such a kind of acoustic wave sensor, for example, Japanese Laid-Open Patent Publication No. 2002-156451 discloses an acoustic wave sensor of reflected wave formula. In the acoustic wave sensor of reflected wave formula, compressional wave such as ultrasonic wave is intermittently transmitted from a wave transmitter having a wave transmitting device to a medium, and reflected wave reflected by an object is received by a wave receiver having a wave receiving device. A distance to the object and an orientation where the object is located is detected on the basis of a time difference from the transmission of the compressional wave to the receiving of the reflected wave.
On the other hand, for example, Japanese Laid-Open Patent Publication No. 2003-279640 discloses an acoustic wave sensor of direct wave formula. In the acoustic wave sensor of direct wave formula, compressional wave is intermittently transmitted from a wave transmitter to a medium, and a distance to the wave transmitter and an orientation where the wave transmitter is located are detected on the basis of a time difference from the transmission of the compressional wave to the receiving of the compressional wave by a wave receiver.
As for the application of the acoustic wave sensor, an ultrasonic level gauge, a back sonar in-vehicle, and so on are provided as examples which propagate the ultrasonic wave in the air. Alternatively, a sonar, a fish detector, and so on are provided as examples which propagate the ultrasonic wave in the underwater. Furthermore, an ultrasonic flaw detector, an ultrasonic CT, and so on are provided as examples which propagate the ultrasonic wave in a structure.
In the above-mentioned acoustic wave sensor, the wave receiver has a plurality of wave receiving devices which are arranged on the same plane for receiving the acoustic waves transmitted from the wave transmitter. When an orientation of arrangement of the wave receiving devices forms a predetermined angle (except cases of right angle and parallel) with respect to an arrival orientation of the acoustic waves corresponding to the orientation where the object is located, time differences occur in times when the wave receiving devices respectively receive the acoustic waves owing to an arrangement pitch of the wave receiving devices and the predetermined angle. Therefore, it is possible to detect the arriving direction of the acoustic wave, that is, the direction where the object is located by detecting phase differences between two signals outputted from adjoining two wave receiving devices when they receive the acoustic waves.
In the conventional acoustic wave sensors, piezoelectric devices are widely used as the wave transmitting device for transmitting acoustic wave in the air and the wave receiving devices for converting received acoustic waves to wave receiving signals of electric signals. In the acoustic wave sensor which uses piezoelectric devices for both of the wave transmitting device and the wave receiving devices, frequency of acoustic wave transmitted from the wave transmitting device is generally set in a frequency near to resonance frequency of the wave transmitting device and the wave receiving devices for a purpose of increasing acoustic pressure of the acoustic wave to be transmitted and sensitivity for sensing acoustic wave in each wave receiving device.
However, in the acoustic wave sensor using the piezoelectric device as the wave transmitting device, reverberation component due to resonance of the wave transmitting device is included in the acoustic wave transmitted from the wave transmitting device. In addition, reverberation component due to resonance of the wave receiving device is included in the wave receiving signal outputted from each wave receiving device in the acoustic wave sensor using the piezoelectric devices as the wave receiving devices. In the acoustic wave sensor using the piezoelectric devices for both of the wave transmitting device and the wave receiving devices, reverberation components due to resonances of both devices are included.
A Q factor (mechanical quality factor Qm) of resonance characteristic of the piezoelectric device is generally larger than 100. Thus, when the wave transmitting device of the piezoelectric device is intermittently driven, the acoustic wave generated by the wave transmitting device becomes vibration wave as shown in FIG. 20. The larger the value of the Q factor of the resonance characteristic becomes, the longer a term T1 necessary for becoming the amplitude of the vibration waveform to the greatest, and the longer a term (reverberation term) T2 necessary for converging the reverberation oscillation become. Thus, the term from the transmission of the acoustic wave to the receiving of the acoustic wave becomes shorter.
Therefore, for example, in the acoustic wave sensor for detecting a distance to an object, it is impossible to detect the distance to the object with respect to the object positioned within a predetermined distance from the wave receiving devices. Hereupon, when a temperature is designated by a symbol “t” (° C.), acoustic velocity “c” (m/s) of acoustic wave is calculated by the equation c=331.5+0.6t. For example, when the temperature is assumed as 14 degrees Celsius, acoustic velocity c is 340 (m/s). In this case, acoustic wave advances only 34 cm per 1 ms. When assuming the reverberation term T2 of the vibration waveform of the acoustic wave transmitted from the wave transmission device is 2 ms, it is impossible to measure the distance to the object located at a position within 34 cm from the wave receiving devices.
As just described, in the acoustic wave sensor using the piezoelectric device as the wave transmitting device, since a dead zone caused by the reverberation component included in the acoustic wave transmitted from the wave transmitting device is longer, a distance to an object relatively near to the wave receiving devices cannot be detected.
Furthermore, it is assumed that two objects are located in relatively shorter distances to the acoustic wave sensor using the piezoelectric devices as the wave transmitting device and the wave receiving devices. Reflected wave reflected by one object may arrive at the wave receiving devices of the acoustic wave sensor while reflected wave reflected by the other object is received by the wave receiving devices. In such a case, it is difficult to distinguish between the reflected waves from these two objects on the basis of the wave receiving signals outputted from the wave receiving devices.
In other words, the acoustic wave sensor using piezoelectric devices for the wave transmitting device and the wave receiving devices has a long dead zone caused by the reverberation component included in the acoustic wave transmitted from the wave transmitting device and the reverberation component included in the wave receiving signals outputted from the wave receiving devices. Thus, even in an area where the distance and/or orientation of the object can be detected by the acoustic wave sensor, when differences among distances from the acoustic wave sensor to a plurality of the objects are smaller, the distances to the objects may not be detected. Accordingly, it is desired to improve angular resolution of the acoustic wave sensor. In addition, the wave transmitting device and the wave receiving devices used in the acoustic wave sensor have characteristics that the Q factor of resonance characteristic becomes larger as the angular resolution becomes lower.
Furthermore, it is possible to constitute a position detecting system for detecting position information of an object to be detected with using the above acoustic wave sensor. For example, according to the above Japanese Laid-Open Patent Publication No. 2003-279640, a position detecting system, which comprises ultrasonic wave transmitters respectively provided on a plurality of migration objects, at least three ultrasonic wave receivers respectively provided in predetermined areas on a ceiling of a building, and a processor for obtaining position information of the migration objects on the basis of a term from a time when the ultrasonic wave transmitter transmits the ultrasonic wave to a time when the ultrasonic wave receiver receives the ultrasonic wave, is proposed.
In such a position detecting system, the ultrasonic wave receivers are provided one by one in every predetermined areas on the ceiling of the building, so that it is necessary to install the ultrasonic wave receivers in at least three places so as to obtain the positioning information of the object to be detected (migration object) in the processor. Furthermore, the position information of the object can be obtained in a domain where sensing areas of three ultrasonic wave receivers are piled up, so that the disposition of the ultrasonic wave receivers is difficult.